Objective: To determine the effect of a protective environment on 1) the control of infection in immunosuppressed recipients of allogeneic marrow grafts, and 2) the incidence and severity of graft-versus-host disease post transplantation. Approach: Patients receiving allogeneic marrow transplants from HL-A and MLC matched siblings as therapy for acute leukemia or aplastic anemia will be randomized into laminar airflow (LAF) or conventional isolation. LAF patients will receive oral non-absorable antibiotics to suppress the gut flora, and will be served sterile food. They will remain in isolation for 50 days or until all signs of graft-versus-host disease have disappeared. Both groups of patients will be monitored for infections and graft-versus-host disease.